Defects in Pyrimidine Degradation Identified by HPLC-Electrospray Tandem Mass Spectrometry of Urine Specimens or Urine-soaked Filter Paper Strips

Background: Urinary concentrations of thymine, uracil, and their degradation products are useful indicators of deficiencies of enzymes of the pyrimidine degradation pathway. We describe a rapid, specific method to measure these concentrations to detect inborn errors of pyrimidine metabolism. Methods: We used urine or urine-soaked filter-paper strips as samples and measured thymine, uracil, and their degradation products dihydrothymine, dihydrouracil, N-carbamyl-β-aminoisobutyric acid, and N-carbamyl-β-alanine. Reversed-phase HPLC was combined with electrospray ionization tandem mass spectrometry, and detection was performed by multiple-reaction monitoring. Stable-isotope-labeled reference compounds were used as internal standards. Results: All pyrimidine degradation products could be measured in one analytical run of 15 min. Detection limits were 0.4–4 μmol/L. The intraassay imprecision (CV) of urine samples with added compounds was 1.3–12% for liquid urines and 1.0–10% for filter-paper extracts of the urines. The interassay imprecision (CV) was 3–11% (100–200 μmol/L). Recoveries were 89–99% at 100–200 μmol/L and 95–106% at 1 mmol/L in liquid urines, and 93–103% at 100–200 μmol/L and 100–106% at 1 mmol/L in filter-paper samples. Correct identifications of deficiencies of the pyrimidine-degrading enzymes were readily made with urine samples from patients with known defects. Conclusions: HPLC with electrospray ionization tandem mass spectrometry allows rapid testing for disorders of the pyrimidine degradation pathway, and filter-paper samples allow easy collection, transport, and storage of urine samples.